Apparatus and method for sterilizing fine powders

Abstract: Apparatus for sterilizing a fine powder comprises a tubular sterilizing duct 13 and a heating jacket for heating the duct 13 to a sterilizing temperature. A feed hopper 11 and feed screw 12 are provided for introducing fine powder at one end of the duct 13 and an insulated hopper 16 is provided for collecting fine powder from the other end of the duct 13. Air introduction means is provided at one end of the duct 13 for fluidising powder in the duct 13. In the method, the duct 13 is heated to a temperature of approximately 500°C and fine powder is introduced at one end of the duct 13 and removed from the other end of the duct 13 with a duct residence time sufficient to heat the powder to above 120°C. Air is introduced into the duct 13 at a rate of 30-60 litres per minute.

Description: This invention relates to an apparatus and method for sterilizing fine powders such as talcum powder. Talcum powder for personal use requires sterilization to kill potentially harmful organisms. One prior art sterilization method comprises placing a bulk quantity of talcum powder in a vacuum chamber, evacuating the air, and subsequently flooding the chamber with ethylene dioxide. This method is effective but has the disadvantage that under certain conditions ethylene dioxide may be explosive; to counter this threat it has become common to mix a quantity of CFC's (chloro-fluoro carbons) with the ethylene dioxide. Current legislation to limit the use of CFC's is rendering this method of sterilization less desirable, and may eventually force manufacturers and processors to cease using the method. Radiation might as an alternative be used to sterilize talcum powder but this method is rather expensive, and requires close control of the process and skilled staff. Another method of sterilization is to use heat, but a heat process must be designed to ensure that no relatively cool pockets of unsterilized material remain, for otherwise the process may serve only to encourage multiplication of dangerous organisms. Furthermore the temperature must not be so great as to cause burning or degradation of the powder. It has been suggested to use an air conveyor to carry powder through a duct sufficiently hot to give effective sterilization. However an airspeed of around fifteen metres per second is required to carry the powder, and this results in an excessively long duct if temperatures are to be kept at a reasonable level. The shorter the duct, the higher the temperature required. In any event sterilization times are measured in seconds rather than minutes, thus requiring very close control of the process if effective sterilization without powder degradation is to be achieved. The apparatus and method of the present invention provides for heat sterilization in an effective manner with a relatively long residence time and without the risk of powder degradation at excessive temperatures. According to one aspect of the invention there is provided apparatus for sterilizing a fine powder and comprising a tubular sterilizing duct, means for heating the duct to a sterilizing temperature, means for introducing fine powder at one end of the duct and for collecting fine powder from the other end of the duct, and air introduction means at one end of the duct for fluidising powder in said duct. In a preferred embodiment the heating means comprises a jacket encircling the duct and capable of heating the duct to above a temperature of 180°C. In another aspect, the invention provides a method of sterilizing fine powders in the aforesaid apparatus and comprising the steps of a) heating the sterilizing duct to a temperature of approximately 500°C; b) introducing fine powder at one end of the duct and removing fine powder from the other end of the duct with a powder residence time in the duct sufficient to heat the powder to above 120°C; c) introducing air into the duct at a rate of 30-60 litres per minute. Preferably the duct is a substantially vertical tube through which fine powder passes downwardly, air being introduced at the base of the tube and passing upwardly to fluidise the powder. The tube preferably has a diameter in the range 75-150 mm and a height in the range 4-5 metres. The apparatus may further include a hopper at the outlet end of said duct to collect hot powder, and optionally a cooler to cool the powder for discharge and further processing. The hopper may be insulated to permit the powder to soak at an elevated temperature. After leaving the hopper, the powder may enter a cooler which may comprise a substantially vertical tube through which the powder passes from top to bottom, and having an external water jacket through which coolant is passed in use. In a preferred embodiment a sterilizing tube and a cooling tube are upright and arranged adjacent one another, the hopper being above the cooling tube and lift means being provided to transfer powder from the base of the sterilizing tube to the hopper. The method may include the further steps of d) permitting the fine powder to soak at an elevated temperature for at least 15 minutes on exit from the sterilizing duct; and e) cooling the fine powder for discharge and further processing. Other aspects of the invention will be apparent from the following description of a preferred embodiment illustrated by way of example in the accompanying diagrammatic sketch of sterilization apparatus for talcum powder. Unsterilized talcum powder is conveyed from silos into a feed hopper (11) in any suitable manner and indicated by arrow A - the feed hopper is typically topped-up periodically on signal from a device sensing low powder level. Powder is transferred from hopper 11 at a controlled rate, for example by-feed screw 12, to the upper end of a substantially upright sterilizing tube 13. The sterilizing tube 13 has a heating jacket of any suitable type, for example containing electrical elements, and is insulated. The base of the sterilizing tube 13 is closed by an end plate 14 having an upwardly facing nozzle therein and connections (not shown) to a source of air under pressure. The direction of air flow is indicated by arrow B. Powder passes from a point just above the base of the sterilizing tube 13 (arrow C) to a discharge controller and elevator 15 of any suitable type, and from the elevator 15 (arrow D) to an insulated hopper 16. The hopper 16 discharges into a substantially upright cooling tube 17 surrounded by a water jacket through which coolant is passed in a counterflow direction. The base of the cooling tube 17 is closed by a second end plate 18 also having a nozzle for connection to a source of air under pressure. The direction of airflow is indicated by arrow E. Powder passes from a point just above the base of the cooling tube 17 (arrow F) to a discharge controller and elevator 19 of any suitable type, and from the elevator 19 (arrow G) to a storage bin 20. In operation the sterilizing tube is maintained substantially full with a powder residence time of about 20 minutes and a discharge temperature of around 220°C for a tube of 100 mm internal diameter and a height of 4.5 metres. The residence time is determined by the discharge controller and elevator 15, and may be varied to suit actual powder temperatures achieved. Powder temperature is monitored by temperature sensors of any suitable kind and it is envisaged that the sterilization process may be automated such that residence time is a minimum for given powder temperatures. Air is bubbled through the sterilizing tube 13 and rises through the powder for discharge at the top. The air fluidises the bed and promotes heat transfer and free particle movement. The rate of air injection, and the injection pressure are not of prime importance to the process provided that they are sufficient to fluidise the column of powder. For a tube of the stated dimensions and talcum powder of 0.074 mm (200 mesh size), an air flow rate of 45 litres per minute at an injection pressure of 2 bar has been found satisfactory. The feed screw 12 tops-up the sterilizing tube 13 under the control of level sensors in the tube 13, or in accordance with the volume drawn off by the controller and elevator 15. The insulated hopper 16 is provided to give a heat soak of around 20 minutes, thus ensuring that the hopper and the contents are fully sterilized. Once sterilised the powder is cooled for discharge and transport. A jacketed cooling tube of any suitable kind may be provided, and the powder may optionally be fluidised to promote even cooling and free powder movement. The rate of powder transfer through the cooling tube is determined by the discharge controller and elevator 19. The storage bins 20 may be of a size and type suitable for transport by road, or alternatively the powder may be discharged into a tanker or silo. A bank of sterilizing tubes may be arranged to feed a single insulated hopper 16, thus retaining the beneficial length/diameter ratio of the sterilizing tube within limits which can be installed in a conventional industrial building, and permitting only a proportion of the sterilizing tubes to be operated at times when demand is low or maintenance is required. The hot powder sterilizes those components of the process with which it comes into contact, and thus special attention is only required for equipment downstream of the insulated hopper 16. Air introduced into the sterilizing tube need not be itself sterilized but should be filtered. Air introduced to the cooling tube must be clean and sterile. In certain circumstances, especially for batch production, the cooling tube may not be required, and the insulated hopper 16 may alternatively be replaced by a conventional sealed silo or sterile tank.

Claims: Apparatus for sterilising a fine powder and comprising a tubular sterilising duct (13), means for heating the duct to a sterilising temperature, means (11,12) for introducing fine powder at one end of the duct and for collecting the fine powder from the other end of the duct, and air introduction means at one end of the duct for fluidising the fine powder in said duct. Apparatus according to Claim 1, wherein the heating means comprises a jacket encircling the duct and capable of heating the duct to above a temperature of 180°C. Apparatus according to Claim 1 or Claim 2, wherein the duct is a substantially vertical tube through which fine powder passes downwardly, air being introduced at the base of the tube and passing upwardly to fluidise the powder. Apparatus according to Claim 3, wherein the tube has a diameter in the range of 75-150 mm and a height in the range of 4-5 metres. Apparatus according to any preceding claim and including a hopper (16) at the outlet end of said duct to collect hot powder. Apparatus according to Claim 5, wherein the hopper is insulated. Apparatus according to Claim 5 or Claim 6 and further including a cooler (17) downstream of said hopper. Apparatus according to Claim 7, wherein the cooler (17) comprises a substantially vertical tube through which the powder passes from top to bottom, and an external water jacket through which coolant is passed in use. Apparatus according to Claim 7 or Claim 8, wherein the sterilising tube and the cooling tube are upright and arranged adjacent one another, the hopper (16) being above the cooling tube (17) and lift means (15) being provided to transfer powder from the base of the sterilising tube to the hopper. A method of sterilising fine powders and comprising the steps of: a) heating a sterilizing duct to a temperature of approximately 500°C; b) introducing fine powder at one end of the duct and removing fine powder from the other end of the duct with a powder residence time in the duct sufficient to heat the powder to above 120°C; c) introducing air into the duct at a rate of 30-60 litres per minute. A method of sterilizing fine powders according to Claim 10 and further including the steps of: d) permitting the fine powder to soak at an elevated temperature for at least 15 minutes on exit from the sterilizing duct; and e) cooling the fine powder for discharge and further processing.

Inventors: KITCHING PETER JEFFREY, KITCHING, PETER JEFFREY
Applicants: COLIN STEWART MINCHEM LTD, COLIN STEWART MINCHEM LTD.
Application year: 1994
Main classification: A61L2
IPC codes: A61L2